It could be dry mouth, infection, Mucocitis, neutropenic ulcers, a combination of these, and others. Drink plenty water, see about getting extra IV hydration at the chemo center, use less harsh toothpaste, no alcohol mouthwash, soft toothbrushes, sponge type or baby brushes, rinsing the mouth, brushing too often with harsh products can cause mouth issues. Rinsing with Glutamine powder helps in repairing tissue, helps neuropathy too. Magic mouthwash, similar products, help ease the pain, antibacterial rinses like chlohexadine help with mucocitis. Juven, made by Abbott pharmaceuticals, has Glutamine in it, and was given to me by my chemo center, which is swallowed, tastes good, repairs the mucosal lining. NeutraSal, a prescription rinse, helps with dry mouth, Mucocitis. My radiation center sprayed my mouth daily with a saline solution, which helped with mucocitis, gets the gunk out, helps controlling thrush. Those home oral cleaning products can be used the same way. Sucking on ice chips may ease the mouth pain. Try to avoid acidic foods, anything else that may irritate the mouth like hot temperature, spicy and sharp textured foods.

I hope this helps, and check with the doctor(s).